Hitherto, there have been no illumination devices which on the one hand in the undimmed state have far higher light yields than an incandescent or halogen incandescent lamp and on the other hand have a color temperature which changes when it is dimmed in a similar way to an incandescent lamp.
FIG. 1 shows the color locus of an incandescent lamp in the undimmed state (locus 1) and in the dimmed state (locus 2). When the incandescent lamp is dimmed, the color temperature is reduced at the same time as the intensity of illumination, the light becomes “warmer”, and the fraction of the light emitted in the red spectral region grows. Therefore, the incandescent lamp has a similar behavior to daylight, which has a lower illumination intensity and a lower color temperature in the morning and evening than during the middle of the day. The human eye and sensitivity are attuned to this. By contrast, the fact that the color temperature of Hg fluorescent lamps remains constant or even rises when the lamps are dimmed is considered unpleasant.
A further problem is that with illumination devices which are fitted with incandescent or halogen incandescent lamps, the luminous color cannot be varied beyond a maximum possible range of the chromaticity diagram. Incandescent or halogen incandescent lamps are not suitable for this purpose, since their light yield is low and their color locus is more or less on the Planckian locus (FIG. 1). The remainder of the chromaticity diagram can only be achieved with the aid of color filters, which would reduce the light yield.
One solution under discussion is that of combining a plurality of Hg fluorescent lamps. With a fluorescent lamp with a mercury discharge, a blue (barium magnesium aluminate:Eu=Hg-BAM), a green (cerium magnesium aluminate:Tb=Hg-CAT) and a red (yttrium oxide:Eu=YOE) phosphor coating and a common electronic ballast which can vary the intensities of the three fluorescent lamps independently of one another, it is possible to set all the color loci in the chromaticity diagram encompassed by the three individual fluorescent lamps (Hg-BAM, Hg-CAT, Hg-YOE) (cf. FIG. 1). However, the drawback of this solution is that Hg fluorescent lamps cannot generate saturated red, green or yellow light, because the Hg low-pressure discharge, in addition to the UV radiation which excites the phosphor, also emits an excessively high level of blue light. Consequently, white light with color temperatures<2600 K, as are generated, for example, when an incandescent lamp is dimmed, cannot be generated.
Therefore, it is an object of the invention to provide an illumination device which avoids the drawbacks listed above.